fix: Add NoneType as a local member to the _common module

[hutcheb]

#32 Provides a consistent way to handle the NoneType. Now even when using pypy it creates a local attribute in the _common module, this is imported in the init.py, where if we don't have attribute but it is declared in the slots then it freaks out.

try:
NoneType
except NameError:
NoneType = type(None)

try:
NoneType = NoneType
except NameError:
NoneType = type(None)

[hutcheb]

There are then some other issue with the tests when using pypy, but they don't seem to be related to this.

It looks like it might be related to the ordering of the R,W,X attributes in the Perm test Flag class. Looks like pypy orders them differently.